A TMR (Tunneling MagnetoResistance) sensor is a new magnetoresistance effect sensor that has begun to be applied to the industry field in recent years. The sensor utilizes the tunnel magnetoresistance effect of a magnetic multilayer film material to sense a magnetic field, which is mainly manifested in: in a magnetic multilayer film material, wherein the resistance of a magnetic multilayer film changes, as the magnitude and direction of an external magnetic field changes. It has a greater magnetoresistance than existing AMR (Anisotropic MagnetoResistance) and GMR (Giant MagnetoResistance) sensors, and it also has better temperature stability than Hall sensors.
Common TMR or GMR push-pull bridge-type sensors require that the magnetization directions of pinning layers of the magnetoresistance sensing elements in two adjacent bridge arm resistors should be oppositely oriented, while, generally for TMR or GMR elements deposited on the same substrate, since the magnitude and direction of the magnetic field are the same, and the magnetization directions of pinning layers of magnetoresistance sensing elements on the same substrate are the same, it is difficult to manufacture a push-pull bridge-type sensor. At present, there are several methods used for manufacturing a push-pull bridge-type sensor on a single chip.
(1) Magnetization directions of pinning layers of magnetoresistance sensing elements in the arm are set in opposite directions by using a two-step film forming process or by local laser assisted magnetic annealing, so as to achieve a single chip bridge-type sensor. The two-step film forming process for depositing the TMR elements, such that the pinning layers are in opposite directions, in two steps respectively. This makes the fabrication process complicated, and a thin film deposited in the first step may be affected during a second annealing process. This makes consistency of the films formed in the two steps poor, thereby affecting the overall performance of the sensor. The local laser heating magnetic annealing method refers to, after initially annealing in the same strong magnetic field, the chip is then local laser annealed, to set the magnetization directions of pinning layers of adjacent arms in opposite directions, thereby achieving a single chip bridge-type sensor. However, this method requires dedicated custom fabrication device which is expensive, and the whole process is time-consuming.
(2) The single chip bridge-type sensor is achieved by tilting the magnetic moment directions of free layers of the magnetoresistance sensing elements in the bridge arms. That is, the magnetization directions of the pinning layers of the magnetoresistance sensing elements within each bridge arm are the same, but the magnetization directions of the free layers of the magnetoresistance sensing elements in adjacent bridge arms are different, but the absolute angle between the magnetization direction of the free layer of each of the magnetoresistance sensing elements and the magnetization directions of its pinning layers is the same. A problem with this method is a reduction of the response of the sensor to the magnetic field, thereby leading to reduction of the sensitivity of the sensor.
(3) Multi-chip packaging technology: two well matched magnetoresistors taken from the same wafer or different wafers, where magnetization directions of pinning layers of the two magnetoresistors are the same, and then in a multi-chip packaging process one of them is rotated by 180 degrees relative to the other magnetoresistor, in order to form a push-pull half bridge. The method can achieve the function of a push-pull type half bridge, that is, the method increases the detection sensitivity and has a temperature compensation function, however, on the other hand, a multi-chip package has a large packaging size and a high production cost. Moreover in actual practice, it is not possible to accurately flip a chip by 180° in the package. As a result, the sensitivity directions of the two resistors are not exactly aligned 180 degrees with respect to each other, and this causes output characteristics of the two resistors to change differently with applied magnetic field. As a result of this asymmetry in sensitivity, there is a greater offset voltage, which may be problematic in many real-world applications.
In addition, the push-pull bridge-type sensor has higher sensitivity than a single-resistor reference bridge-type sensor, it has better temperature compensation, and therefore it can reduce the effect of temperature drift. Prior art push-pull bridge-type magnetic field sensors use a permanent magnet to bias the magnetization directions of the magnetoresistance elements, and the sensor has a higher cost and greater offset, and it is not suitable for high-intensity magnetic fields.